Objective: This study was designed to study the effect of vitaminC on training efficiency in rats and in humans.

Design: The human study was double-blind and randomized. Fourteenmen (27–36 y old) were trained for 8 wk. Five of the menwere supplemented daily with an oral dose of 1 g vitamin C.In the animal study, 24 male Wistar rats were exercised under2 different protocols for 3 and 6 wk. Twelve of the rats weretreated with a daily dose of vitamin C (0.24 mg/cm2 body surfacearea).

Results: The administration of vitamin C significantly (P =0.014) hampered endurance capacity. The adverse effects of vitaminC may result from its capacity to reduce the exercise-inducedexpression of key transcription factors involved in mitochondrialbiogenesis. These factors are peroxisome proliferator–activatedreceptor co-activator 1, nuclear respiratory factor 1, and mitochondrialtranscription factor A. Vitamin C also prevented the exercise-inducedexpression of cytochrome C (a marker of mitochondrial content)and of the antioxidant enzymes superoxide dismutase and glutathioneperoxidase.

Vitamin C modulates endurance capacity but not maximal oxygen uptake after training
The maximal rate of oxygen consumption (O2max)increased significantly after 8 wk of training in both the nonsupplementedmen (22.0% increase) and the men supplemented with vitamin C(10.8% increase). In 1999, Nielsen et al (29) found no effectof antioxidant supplementation on O2maxin triathletes. We found a very similar result in our animalstudy—ie, a significant increase in O2maxafter 6 wk of training in both the nonsupplemented (17.0% increase)and the vitamin C–supplemented (4.7% increase) groups.Endurance capacity is dependent mainly on the mitochondrialcontent of skeletal muscle (muscle oxidative capacity), noton the cardiovascular factors previously mentioned (20). Forobvious ethical reasons, we could not perform an endurance laboratorytest in our volunteers. Thus, to determine the effect of theantioxidant administration and exercise in the mitochondrialmuscle content, we performed another series of experiments inrats. We divided our animals into 2 training groups: endurance-trainedfor 3 wk and endurance-trained for 6 wk. Six weeks is approximatelythe period required to achieve a new steady state mitochondrialcontent in response to endurance training (15), although changesin mitochondrial protein and mRNA content can be apparent atmuch earlier time points (22). In our study, endurance-trainedrats showed a clear increase (186.7%) in their endurance capacity.However, the administration of vitamin C dramatically decreasedthis adaptation to only 26.5%. This finding is in keeping witha previous study in which it was shown, using endurance-trainedrats, that O2max increased only 14%despite a 100% increase in muscle oxidative capacity (21). Oneof the main conclusions from that study was that the mitochondrialcontent of muscle is a major determinant of endurance capacity,whereas the maximal aerobic workload capacity appears to beregulated by O2max (21). We offera molecular explanation for this result (ie, that vitamin Cdecreases exercise-induced mitochondrial biogenesis and theantioxidant capacity in skeletal muscle). We have found thatexercise training up-regulates the following mitochondriogenicpathway: PGC-1 NRF-1 mTFA cytochrome C. All of these adaptationsare prevented by vitamin C administration.

When supplementing with vitamin C, there is the possibilitythat it may act as a prooxidant in vivo. These prooxidativereactions of vitamin C readily occur in vitro, and it has beenshown that they also may have relevance in vivo (30). A highintake of iron along with ascorbic acid could increase in vivolipid peroxidation of LDL and therefore could increase the riskof atherosclerosis (31). However, another study showed that,in iron-overloaded plasma, ascorbic acid acts as an antioxidantand prevents oxidative damage to lipids in vivo (32). In thepresent study, we measured different variables of oxidativestress, eg, blood glutathione oxidation and plasma malondialdehyde,in rats and men (data not shown); we did not find an indicationof an in vivo prooxidant effect of vitamin C in any of the experimentalgroups.

Free radicals as signals in muscle cell metabolism: potential interference by antioxidant vitamins
It is important to consider that free radicals are not alwaysdamaging to cells; in many cases, they serve as signals to adaptmuscle cells to exercise via modulation of gene expression (9,33). We have found that training causes an increase in 2 majorantioxidant enzymes (Mn-SOD and GPx) in skeletal muscle. Wewere surprised to see that vitamin C prevents these beneficialeffects of training. On the basis of the paradigm that enzymaticantioxidant systems such as Mn-SOD and GPx provide a first-linedefense against ROS, it is expected that exercise may inducethese protective mechanisms. Moderate exercise increases lifespan and decreases disability in rats (12) and humans (15).We report here that exercise training causes an increase inthe expression of antioxidant enzymes, which is prevented bythe administration of vitamin C.

Moderate exercise as an antioxidant
A major conclusion that can be drawn from our experiments isthat exercise itself is an antioxidant, because training increasesthe expression of 2 antioxidant enzymes related with longevity—namely,SOD and GPx. We provide evidence that the continuous presenceof small stimuli, such as low concentrations of ROS, in factinduces the expression of antioxidant enzymes as a defense mechanism.Low concentrations of radicals may be considered to be beneficial,because they act as signals to enhance defenses, rather thanbeing deleterious, as they can be when they are at higher concentrations.

Antioxidant vitamins impair training efficiency
The second major conclusion that can be drawn from our experimentsis that supplementation with vitamin C lowers training efficiency.Endurance capacity is directly related to the mitochondrialcontent. This variable is seriously hampered by antioxidantsupplementation, whereas O2max, whichis dependent also on the cardiovascular system adaptations,is not significantly affected. This information is helpful fornutritionists who must prepare diets for athletes whose performanceis dependent on their endurance capacity. It should be takeninto account that some of the world's best marathon runnersexhibit rather modest measures of O2max(34). Antioxidant supplementation is very popular among athletes,but data showing any beneficial effects on muscle function ofthis type of widespread practice are elusive. In fact, severalreports have shown deleterious effects of antioxidant treatment.As early as 1971, it was shown that vitamin E supplementation(400 IU/d for 6 wk) caused unfavorable effects on enduranceperformance (35). In 1996 and 1997, a Scandinavian journal published2 reports showing the deleterious effects of ubiquinone-10 supplementationon the performance of humans after a high-intensity trainingprogram (36, 37). In 2001, Coombes et al (38) reported that,in the muscles of unfatigued rats, supplementation with vitaminE and -lipoic acid depressed muscle tetanic force at low stimulationfrequencies. One year later, it was shown that supplementationof racing greyhounds with 1 g vitamin C/d for 4 wk significantlyslowed their speed (39). Taking into account that a high fitnesslevel is associated with a lower risk of premature death fromany cause, the effect of vitamin C administration on endurancecapacity has important implication for nutritionists, physicians,and exercise trainers and practitioners. Thus, the common practiceof taking vitamin C supplements during training (for both health-relatedand performance-related physical fitness) should be seriouslyquestioned.

If this holds up it should cause many of us to rethink supplementation (or at least certain supplements). Any counter science out there?